Storage batteries are employed in many applications requiring electrical energy to be retained for later use. Most commonly, they are employed in motor vehicles utilizing internal combustion engines. In such applications, energy is stored by "charging" the battery during engine operation and is later used to power lights, radio, and other electrical apparatus when the engine is stopped. The most severe demand upon the battery of a motor vehicle is generally made by the starter motor. Failure to supply the stator motor with sufficient power to satisfactorily crank the engine, particularly in cold weather, is usually the first induction of battery deterioration or trouble with the charging system. Clearly, a simple measurement that accurately assesses a battery's ability to supply power to a heavy load is of considerable value.
Prior to the publication of U.S. Pat. Nos. 3,873,911 and 3,909,708, the only method generally available for accessing a battery's ability to supply load power was the standard load test. A standard load test subjects a battery to a heavy dc load current having a predetermined value dictated by the battery's rating. After a prescribed time interval, the battery's terminal voltage under load is observed. The battery is then considered to have "passed" or "failed" the load test according to whether its terminal voltage is greater than, or less than, a particular value.
Although the standard load test has been widely used for many years to field-test storage batteries, it possesses several serious disadvantages. These disadvantages include:
1. A standard load test draws very large currents and therefore requires apparatus that is heavy and cumbersome.
2. Because of these large currents, considerable "sparking" will occur at the battery terminals if the test apparatus is connected or disconnected under load conditions. Such "sparking" in the presence of battery gasses can cause an explosion with potentially serious injury to the operator.
3. A standard load test leaves the battery in a significantly reduced state of charge and therefore less capable of cranking the engine than before the test was performed.
4. The battery's terminal voltage drops with time during performance of a load test. Accordingly, load tests results are generally imprecise and greatly dependent upon the skill of the operator.
5. Load test results are not repeatable since the test itself temporarily polarizes the battery. Such test-induced polarization significantly alters the initial conditions of any other load tests performed subsequently.
A practical alternative to the standard load test is taught in U.S. Pat. No. 3,873,911, U.S. Pat. No. 3,909,708, and U.S. patent application Ser. No. 169,858. These documents disclose electronic apparatus for accurately assessing a battery's condition by means of small-signal ac measurements of its dynamic conductance. They teach that a battery's dynamic conductance is directly proportional to its load test current; or to its dynamic power--the maximum power that the battery can deliver to a load. Dynamic conductance is therefore a direct measure of a battery's electrical condition.
Two electronic battery tester embodiments are disclosed in U.S. Pat. No. 3,873,911; each of which accurately determines a battery's dynamic conductance and provides the operator with a numerical reading in battery meeasuring units that are directly proportional to this quantity. The first embodiment comprises a bridge circuit that is brought to balance by the operator to obtain the numerical reading. The preferred second embodiment provides the operator with a direct readout that may be displayed numerically on a digital or analog meter. The operating principles of the preferred, direct-reading, second embodiment of the invention taught in U.S. Pat. No. 3,873,911 are based upon the theory of high-gain feedback amplifiers.
U.S. Pat. No. 3,909,708 likewise discloses two electronic battery tester embodiments. However, from the operator's point of view, their operation more closely resembles the operation of a traditional load-test apparatus than does operation of either of the numerical-reading embodiments disclosed in U.S. Pat. No. 3,873,911. Rather than obtaining a numerical measurement, the operator manually sets a selector knob on the panel of the apparatus to the electrical rating of the battery undergoing test. The disclosed apparatus then employs small-signal measurements of dynamic conductance to simply ascertain whether or not the battery is capable of delivering an amount of power appropriate to the selected rating. Accordingly, the two embodiments disclosed in U.S. Pat. No. 3,909,708 provide qualitative "pass-fail" battery condition information, just as does a conventional load test apparatus. However, they accomplish this result without drawing significant current from the battery and are therefore not subject to the serious disadvantages of a load test. Just as with the second embodiment disclosed in the earlier patent, the operating principles of the second, preferred, embodiment disclosed in U.S. Pat. No. 3,909,708 are based upon the theory of high-gain feedback amplifiers.
The improved electronic battery testing device disclosed in U.S. patent application Ser. No. 169,858 incorporates the functions of both of the two earlier-disclosed feedback-type electronic battery testing devices in a single embodiment. By using all fixed resistances and calibrating the output display in appropriate battery measuring units, the disclosed device emulates a direct reading battery tester. In addition, by letting one resistance be variable, calibrating it in appropriate battery rating units, and arranging the output display to denote qualitative conditions, the "pass-fail" type of dynamic conductance battery tester is emulated. A two-position switch is employed to select either a fixed resistance or a variable resistance and hence to select the emulation of either the direct-reading battery testing device or the "pass-fail" type of battery testing device.
In comparison with the load test method of battery appraisal, the dynamic conductance testing method has many advantages. For example, dynamic conductance testing utilizes electronic apparatus that is small and lightweight, draws very little current, produces virtually no sparking when connected or disconnected, does not significantly discharge or polarize the battery, and yields very accurate, highly reproducible, test results. Virtually millions of battery measurements performed over the course of thirteen years have fully corrobrated these teachings and have proven the validity of this alternative testing method.